The present invention relates to ArF and KrF excimer laser apparatus and fluorine laser apparatus for lithography. More particularly, the present invention relates to gas laser apparatus for lithography, e.g. ArF excimer laser apparatus, KrF excimer laser apparatus, and fluorine laser apparatus, which perform a lasing operation with a long laser oscillation pulse width.
With the achievement of small, fine and high-integration semiconductor integrated circuits, it has been demanded that projection exposure systems for the manufacture of such highly integrated circuits be improved in resolution. Under these circumstances, the wavelength of exposure light emitted from light sources for lithography is becoming shorter. At present, KrF excimer laser apparatus are used as light sources for lithography. ArF excimer laser apparatus and fluorine laser apparatus are promising as next-generation light sources for semiconductor lithography.
In these excimer laser apparatus, a laser gas is sealed in a laser chamber under several hundred kPa. That is, in the KrF excimer laser, a mixed gas of fluorine (F2) gas, krypton (Kr) gas and a rare gas, e.g. neon (Ne), as a buffer gas is sealed in the laser chamber as a laser gas. In the ArF excimer laser, a mixed gas of fluorine (F2) gas, argon (Ar) gas and a rare gas, e.g. neon (Ne), as a buffer gas is similarly sealed in the laser chamber as a laser gas. In the fluorine laser, a mixed gas of fluorine (F2) gas and a rare gas, e.g. neon (Ne), as a buffer gas is similarly sealed in the laser chamber as a laser gas. In these apparatus, the laser gas as a laser medium is excited by generating an electric discharge in the laser chamber.
These laser apparatus emit laser beams having a wide spectral linewidth. Therefore, in order to avoid the problem of chromatic aberration in the projection optical system mounted in the exposure system, it is necessary that the spectral linewidth be narrowed down to 1 pm or less. Narrowing of the spectral linewidth is realized by placing a line-narrowing optical system comprising, for example, a magnifying prism and a diffraction grating, in the laser resonator.
Incidentally, the ArF excimer laser apparatus have an oscillation center wavelength of 193.3 nm, which is shorter than the oscillation center wavelength of the KrF excimer laser apparatus presently used as light sources for lithography, i.e. 248 nm. Accordingly, quartz used as a vitreous material in the projection lens system of a stepper or other exposure system is damaged to a larger extent than in the case of using KrF excimer laser apparatus, resulting in a reduction in lifetime of the lens system.
Damage to quarts includes color-center formation due to two-photon absorption and a compaction (an increase in refractive index). The former appears as a reduction in transmittance, and the latter as a reduction in resolution of the lens system. The influence of the damage is in inverse proportion to the laser pulse width (Tis), which is defined by the following equation, when the laser pulse energy is assumed to be constant:
Tis=(∫T(t)dt)2/∫(T(t))2dtxe2x80x83xe2x80x83(1) 
where T(t) is a temporal laser pulse shape.
Let us describe the definition of the laser pulse width Tis. Assuming that an optical element is damaged by two-photon absorption, because the damage is proportional to the square of the laser light intensity, the damage D accumulated per pulse is given by
D=kxc2x7∫(P(t))2dtxe2x80x83xe2x80x83(2) 
where k is a constant determined by a substance, and P(t) is a temporal laser intensity (MW).
The laser intensity P(t) may be separated into time and energy by the following equation:
P(t)=Ixc2x7T(t)/∫T(txe2x80x2)dtxe2x80x2xe2x80x83xe2x80x83(3) 
where I is energy (mJ), and T(t) is a temporal laser pulse shape.
Temporally integrating P(t) gives I. In the case of ArF excimer laser, I is 5 mJ, for example.
If Eq. (3) is substituted into Eq. (2), the damage D is expressed by
D=kxc2x7I2xc2x7∫(T(t)/∫T(txe2x80x2)dtxe2x80x2)2dt =kxc2x7I2xc2x7∫(T(t))2dt/(∫T(t)dt)2xe2x80x83xe2x80x83(4) 
Substituting Eq. (1) into Eq. (4), we obtain
D=kxc2x7I2/Tisxe2x80x83xe2x80x83(5) 
From Eq. (5), the pulse width Tis, which is in inverse proportion to the damage D, is defined by Eq. (1) because kxc2x7I2 is constant (I is maintained at a constant value).
There have heretofore been cases where the laser pulse width is defined by the full width at half maximum (FWHM) of the temporal laser pulse shape. When the laser pulse width is defined by the full width at half maximum, different temporal laser pulse shapes may become equal to each other in laser pulse width as shown in the model diagram of FIG. 8. In the example shown in FIG. 8, however, the actual laser pulse durations of the two temporal laser pulse shapes are different from each other. That is, the pulse duration of the triangular laser pulse shape is longer than that of the rectangular laser pulse shape. Meanwhile, in the case of the laser pulse width Tis defined by Eq. (1), the laser pulse width Tis of the triangular laser pulse shape shown in FIG. 8 is longer than that of the rectangular laser pulse shape. In the example shown in FIG. 8, for instance, the laser pulse width Tis of the triangular laser pulse shape is twice as long as the laser pulse width Tof the rectangular laser pulse shape.
As has been stated above, the reduction in transmittance due to two-photon absorption and the reduction in resolution due to a compaction are in inverse proportion to the laser pulse width Tis, which is given by Eq. (1), when the laser pulse energy is assumed to be constant. Therefore, it is demanded that the laser pulse width Tis be stretched (i.e. a longer pulse width should be achieved).
Narrow-linewidth ArF excimer laser apparatus for lithography commercially available at present in general perform an oscillating operation at a repetition frequency (hereinafter referred to as xe2x80x9crepetition ratexe2x80x9d) of 1 kHz and provide a laser output of 5 W. In order to avoid damage to the optical system mounted in the semiconductor exposure system, it is necessary that the laser pulse width Tis be 30 ns or longer.
As has been stated above, it is demanded in ArF excimer laser apparatus that the laser pulse width Tis be stretched to achieve a longer pulse width in order to reduce the damage to the optical system mounted in the exposure system. The achievement of a longer pulse width is also demanded for KrF excimer laser apparatus and fluorine laser apparatus from the following points of view.
In a projection exposure system, an image of a mask provided with a circuit pattern or the like is projected through a projection lens onto a work, e.g. a wafer, coated with a photoresist. The resolution R of the projected image and the depth of focus DOF are expressed by
R=k1xc2x7xcex/NAxe2x80x83xe2x80x83(6) 
DOF=k2xc2x7xcex/(NA)2xe2x80x83xe2x80x83(7) 
where k1 and k2 are coefficients reflecting the characteristics of the resist and so forth; xcex is the wavelength of exposure light emitted from a light source for lithography; and NA is a numerical aperture.
To improve the resolution R, the wavelength of exposure light is reduced, and the NA is increased, as will be clear from Eq. (6). However, the depth of focus DOF decreases correspondingly, as shown by Eq. (7). Consequently, the influence of chromatic aberration increases. Therefore, it is necessary to further narrow the spectral linewidth of exposure light. In other words, it is demanded that the spectral linewidth of the laser beam emitted from the gas laser apparatus for lithography be further narrowed.
It is stated in Proc. SPIE Vol. 3679. (1999) 1030-1037 that according as the laser pulse width increases, the spectral linewidth of the laser beam narrows. This was actually proved by an experiment conducted by the present inventors. In other words, it is demanded in order to improve the resolution R that the spectral linewidth of the laser beam be further narrowed. To meet this demand, it is necessary to stretch the pulse width of the laser beam.
Thus, it has become essential to stretch the laser pulse width Tis in order to avoid damage to the optical system in the exposure system and to improve the resolution. It is known that the laser pulse width Tis depends upon the concentration of fluorine gas in the laser gas sealed in the laser chamber [see the above-mentioned Proc. SPIE Vol. 3679. (1999) 1030-1037]. By adjusting the fluorine gas concentration, the laser pulse width Tis can be stretched to achieve a longer pulse width, i.e. Tisxe2x89xa730 ns.
In Japanese Patent Application No. Hei 11-261628, the present inventors propose a method of forming a laser pulse of Tisxe2x89xa730 ns by performing a laser oscillating operation by the first half-cycle of the discharge oscillating current waveform of one pulse in which the polarity is reversed, together with at least one half-cycle subsequent to the first half-cycle.
There have been demands that KrF excimer laser apparatus presently used as light sources for semiconductor lithography, and ArF excimer laser apparatus or fluorine laser apparatus, which are promising as next-generation light sources for semiconductor lithography, should achieve a higher resolution and a higher throughput and reduce the damage to quartz.
However, the technique of achieving a longer pulse width for obtaining a higher resolution and reducing the damage effectively and the technique of achieving a higher repetition rate to obtain a higher throughput are contrary to each other from the viewpoint of the capability of sustaining stable electric discharge. Accordingly, it has been deemed difficult for the two techniques to be compatible with each other. It is reported that it is difficult to achieve a longer pulse width in gas laser apparatus using fluorine, in particular [for example, see Mitsuo Maeda xe2x80x9cExcimer Laserxe2x80x9d, p. 163, and IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS VOL. 5, No. 6 (1999), p. 1515].
The present invention was made in view of the above-described circumstances. An object of the present invention is to provide an ArF excimer laser apparatus for lithography capable of stretching the laser pulse width even when the repetition rate exceeds 4 kHz and also provide a KrF excimer laser apparatus and fluorine laser apparatus for lithography capable of stretching the laser pulse width even when the repetition rate exceeds 2 kHz.
To attain the above-described object, the present invention provides ArF and KrF excimer laser apparatus and fluorine laser apparatus for lithography, each having a pair of laser discharge electrodes connected to output terminals of a magnetic pulse compression circuit and disposed in a laser chamber. A peaking capacitor is connected in parallel to the pair of laser discharge electrodes.
The capacities of capacitors and the inductances of circuit loops in the laser apparatus are denoted as follows: The capacity of a capacitor in the final stage of the magnetic pulse compression circuit is denoted by Cn (n is the number of stages of the magnetic pulse compression circuit); the capacity of the peaking capacitor is denoted by Cp; the inductance of a first circuit loop formed by the capacitor in the final stage and the laser discharge electrodes is denoted by Ln; and the inductance of a second circuit loop formed by the peaking capacitor and the laser discharge electrodes is denoted by Lp.
The relationship between the period Tn of the waveform of an oscillating current flowing in the first circuit loop, i.e. Tn=2xcfx80{square root over ( )}(Lnxc3x97Cn), and the period Tp of the waveform of an oscillating current flowing in the second circuit loop, i.e. Tp=2xcfx80{square root over ( )}(Lpxc3x97Cp), satisfies the following condition:
5Tpxe2x89xa6Tn 
In addition, the period Tn satisfies the following condition:
Tn less than 250 ns 
With the above-described arrangement, a laser oscillating operation is performed by at least 2.5 cycles of the oscillating current flowing between the laser discharge electrodes.
In addition, the present invention provides ArF and KrF excimer laser apparatus and fluorine laser apparatus for lithography, each having a pair of laser discharge electrodes connected to output terminals of a magnetic pulse compression circuit and disposed in a laser chamber. A peaking capacitor is connected in parallel to the pair of laser discharge electrodes.
The capacities of capacitors and the inductances of circuit loops in the laser apparatus are denoted as follows: The capacity of a capacitor in the final stage of the magnetic pulse compression circuit is denoted by Cn (n is the number of stages of the magnetic pulse compression circuit); the capacity of the peaking capacitor is denoted by Cp; the inductance of a first circuit loop formed by the capacitor in the final stage and the laser discharge electrodes is denoted by Ln; and the inductance of a second circuit loop formed by the peaking capacitor and the laser discharge electrodes is denoted by Lp.
The relationship between the period Tn of the waveform of an oscillating current flowing in the first circuit loop, i.e. Tn=2xcfx80{square root over ( )}(Lnxc3x97Cn), and the period Tp of the waveform of an oscillating current flowing in the second circuit loop, i.e. Tp=2xcfx80{square root over ( )}(Lpxc3x97Cp), satisfies the following condition:
3Tpxe2x89xa6Tn less than 5Tp 
In addition, the period Tn satisfies the following condition:
Tn less than 250 ns 
With the above-described arrangement, a laser oscillating operation is performed by at least 1.5 cycles of the oscillating current flowing between the laser discharge electrodes.
In the above-described laser apparatus, it is desirable that the laser discharge electrodes should have a length of 600 to 750 mm and a gap of 15 to 18 mm, and the overall gas pressure in the laser chamber should be 2 to 4 atmospheric pressure, and the fluorine concentration in the laser chamber should be not more than 0.15%, and further the capacity Cn of the capacitor in the final stage should be not less than 8 nF.
Further, it is desirable that a capacitor for preionization should be connected in parallel to the peaking capacitor and in series to corona preionization electrodes, and the combined capacity Cc of the electrostatic capacity of the corona preionization electrodes and the capacity of the capacitor for preionization should be not more than 5% of the capacity Cp of the peaking capacitor.
Further, it is desirable that the output mirror of the optical resonator should have a reflectance of not less than 40%.
Thus, according to the present invention, the primary current for injecting energy into the discharge electrodes from the magnetic pulse compression circuit through the peaking capacitor and the secondary current for injecting energy into the discharge electrodes from the peaking capacitor-charging capacitor in the final stage of the magnetic pulse compression circuit are superimposed on one another. The oscillation period of the secondary current is set to at least 5 times, or not less than 3 times and less than 5 times, as long as the oscillation period of the primary current, and the oscillation period of the secondary current is set to less than 250 ns, whereby a laser oscillating operation for each pulse is performed by the first half-cycle of the waveform of the primary discharge oscillating current, on which the secondary current is superimposed, together with at least four half-cycles or two half-cycles subsequent to the first half-cycle. Accordingly, it is possible to realize a high-repetition rate and narrow-linewidth ArF excimer laser apparatus for semiconductor lithography capable of operating stably with a stretched pulse width even when the repetition rate is 4 kHz or higher. In addition, it is possible to realize high-repetition rate and narrow-linewidth KrF excimer laser apparatus and fluorine laser apparatus for semiconductor lithography capable of operating stably with a stretched pulse width even when the repetition rate is 2 kHz or higher.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.